sia 
104 O. N. Rood on Fresnel’s Theory, and on a mode 
gin of the appearances in all three of the cases is ascribable to 
the same cause: viz., to a wave-like, vertical motion produc 
by the air on the surface of nearly still water, combined with 
feeble horizontal currents (acting in directions nearly at right 
angles to each other) upon the bottom. 
Amherst College, Oct. 15, 1866. 
Art. XV.—On a Theory proposed by Fresnel, and on a mode of 
measuring the average size of very fine particles; by OcpEn N. 
Roop, Prof. of Physics in Columbia College. 
Ir the light from a candle-flame be received on a ground glass 
surface, so obliquely that the incident ray makes only a very 
small angle with the glass surface, the light will be copiously re- 
flected, and a bright uncolored image of the flame will be seen 
by reflexion. As the angle made by the incident ray is increased, 
the reflected image becomes first yellow, then red, and finally: 
disappears altogether. 
Fresnel has attempted to account* for this fact, on the ground 
that the more refrangible rays, having shorter wivelobpahk are 
caused to interfere by a difference of path, which is still too small 
to effect complete interference in the case of the longer waves of 
red light; the difference in path, depending on the iepth of the 
minute scratches on the surface of the glass, and on the angle 
which the ray makes with this surface. 
As it is not difficult to measure approximately the angle at 
which the red ray ceases to be reflected, it would be easy to put 
this theory to the test of experiment, if the average depth of the 
scratches on the ground surface were known. 
The impossibility of obtaining such measurements, has hitherto 
prevented this theory of the action of finely roughened surfaces 
on light, from being either confirmed or overthrown. 
* Pogg. Annalen, Bd. xii, p. 210. 
